I. Field of the Invention
The invention relates to a method for control of an automatic transmission of a vehicle in a downhill-travel situation.
II. Description of Related Art
Vehicles equipped with automatic transmission, particularly hydraulically controlled transmission, have long suffered from a deficiency that becomes evident during downhill-travel situations. In fact, when the driver takes his foot off the accelerator, the vehicle adopts the longest transmission ratio or disengages every transmission ratio. As a result, the vehicle cannot benefit from the braking effect of the engine in a downhill-travel situation in order to govern the speed of the vehicle.
To remedy this shortcoming, there was proposed in European Patent 1041314 a method for control of an automatic transmission wherein the slope on which the vehicle is traveling is detected, in which case a specific rule for control of the vehicle is adopted. This rule, stored in memory in the form of a map, modifies the choice of transmission ratios in order to achieve the braking effect of the engine. It establishes a correspondence among the speed of the vehicle, the power demanded and the transmission ratio.
However, this transitional rule is adapted only to a medium downhill-travel situation. In the case of a steep slope, where the braking effect of the engine would have to be very strong, the ratio chosen is generally too long to achieve the braking effect of the engine. Similarly, in the case of a very gentle slope, the braking effect of the engine is too great and the vehicle slows down.